Forearm strength is a great game improvement secret. Roger Federer confesses that he is blessed with a strong wrist. That's where he gets all that rackethead speed. A strong wrist comes from a strong forearm, so build up that forearm. Here's how:
Grip-Master (V-shaped spring-loaded device that strengthens your grip - I love it, done wonders for my game). Do it by counts or by time. Do it till you can't really hold something afterwards (but not before a tennis game 'cos your muscles will be too fatigued to perform). You'll be amazed at how light your racket feels with just a couple of sessions with it.
As an alternative, use an old tennis ball. Keep pressing it until your forearms feel like jelly. Do this often and you will build a strong grip, wrist and forearm. Very essential for power and racket control. After this I guarantee more zing and kick in your every stroke.
Shorten your backswing.
This is the BEST CURE for most players. Think 1ft backswing, 2ft follow-through. The pros are always trying to make their swings more 'compact'. This is exactly what it means. It improves timing and control, which improves power and accuracy.
I cannot emphasize enough that too many players have too long a backswing. The backswing is the most unnecessary part of your stroke. The more time you spend taking the racket back, the less time you have moving that racket towards the ball.
Meet the ball with your racket face perpendicular to the ground. It is sometimes called 'hitting with a flat face'. Spin is produced by how you swing at the ball, not by the angle of your racket face.
Practice getting your racket in position directly behind the ball. Ideally, you want to be hitting the ball in a swinging motion that is parallel to the ground (this applies to strong players with faster swings). If you like taking the ball waist-height, prepare your racket waist-height or just slightly below it. Then strike through towards your target in a single forward blow. Remember, prepare your racket at the height that you anticipate meeting the ball.
If you are a beginner, or not a fast swinger, swing the racket low-to-high and finish over the opposite shoulder.
Take the racket back by turning your shoulder instead of letting you arm 'reach' backwards. It ensures a compact backswing and promotes meeting the ball in front of you. This applies critically to volleys.
Take bigger steps.
Unless you are very athletic, avoid hitting with an open stance. It is for advance players who play against advance players and have little time to get into the proper hitting position.
SPLIT-STEP. Everytime your opponent hits the ball. It helps you snap into action.